1. Field of the Invention
The present invention relates to DC to DC converters for converting a DC input potential to a DC output potential. More particularly, the present invention relates to DC to DC converters which produce a highly regulated output potential from an input potential which has substantial variations.
2. Description of the Prior Art
Many forms of DC to DC converters are known for converting a DC input power to a DC output power. One well known type of DC to DC converter utilizes an electronically controlled switch connected in series with an inductance with the on time of the switch being modulated by a pulse width modulator. The output potential is proportional to the duration of the pulses applied to the control terminal of the switch by the pulse width modulator.
When this type of DC to DC converter obtains its input power from a battery which is also used for electrically starting engines, such as a gas turbine powered auxiliary power unit, the normal 24 volt battery potential can drop as low as 8 volts, when starting current for an electric starter for an electric starter is being drawn. It is desirable to have a DC to DC converter which produces an output which is not greatly influenced by the input voltage transients of this type.
In a DC to DC converter, the pulse width modulator has commonly been continually controlled exclusively by a feedback signal derived from the output potential. In these converters, the width of the pulses outputted by the pulse width modulator is directly controlled by the feedback signal. Feedback systems typically have circuitry to prevent loop lockup which occurs when the duty cycle of the pulses outputted by the pulse width modulator goes to 100% preventing normal operation and requiring shut down of the converter to return control to the pulse width modulator.
The assignee of the present invention has offered for sale a DC to DC converter more than one year ago which generates the control signal for continually modulating the width of the pulses from the pulse width modulator totally from feed forward of the input potential. In order to insure a high degree of regulation with respect to supplied power, it was necessary to utilize slope and intercept potentiometers to maximize regulation under conditions in which both the input potential and the load applied to the output varied. This power supply exhibited acceptable regulation but was subject to tedious adjustment of the aforementioned potentiometers. Moreover, the potentiometers added expense to the overall cost of the power supply.
DC to DC converters are also known which have a pulse width modulator controlled by control signal which has a majority of control derived from feedback and a minority of control derived from the derivative of the input potential.
Exemplary of DC to DC converters using feedback as the total control mechanism of the pulse width modulator are U.S. Pat. Nos. 4,371,917 and 4,580,089, and West German Patent DE No. 3,340,395.
U.S. Pat. No. 4,618,812 discloses a pulse width modulator having a pulse width modulator controlled by a signal derived from the input potential and signal fed back from the output. The duty cycle of the pulse width modulator is not directly controlled by the input potential. The feed forward signal controls the conduction of transistors which gate the output of the pulse width modulator.
IBM Technical Disclosure Bulletin, Vol. 27, No. 1A, June 1984, pp. 109-114, discloses a DC to DC converter having a pulse width modulator which is controlled totally from sensing of the magnitude of the input signal.